The present invention relates to a drawing communication apparatus and, more specifically, to a drawing communication apparatus which allows a plurality of terminal devices to share display contents.
In recent years, to smoothly establish mutual understanding with a person who is in a distant location, computer systems have been constructed which allow a plurality of terminal devices to share display contents through a network, or the like. In these computer systems, drawing data as an object of writing and erasure is shared by at least two terminal devices.
In a conventional drawing communication apparatus which allows sharing of drawing data that is input by, for instance, hand writing, a control method is employed which permits only one terminal device to input data at a time. However, this control method needs complex control processing in determining an input-permitted terminal device among a number of terminal devices.
To solve this problem, there has been proposed, for instance, a drawing communication apparatus disclosed in Japanese Unexamined Patent Publication No. Hei. 4-241054. This drawing apparatus has two kinds of drawing means, i.e., writing-type and erasure-type drawing means. A terminal device stores writing-type drawing data, and when receiving erasure-type drawing data from another terminal device, it performs erasure processing after redrawing writing-type drawing data that have occurred after occurrence of the previous erasure-type drawing data. This apparatus permits a plurality of terminal devices to input data at the same time. That is, the above problem is solved by redrawing an accumulated difference when receiving erasure-type drawing data or a signal indicating completion of an erasure-type drawing mode (pen off).
For example, as shown in FIG. 1, if a region b is erased from a drawing image of a first terminal device at T11 and then a vertical line segment c is written to a drawing image of a second terminal device at time T12, there occurs, at time T14, a difference between a drawing image 11 of the first terminal device and a drawing image 21 of the second terminal device. The difference occurs at a portion of the line segment c that belongs to the region b. Subsequently, if an oblique line segment d is written to the drawing image of the second terminal device at time T15 and immediately thereafter (at time T16) a region e of the drawing image is erased from the drawing image of the first terminal device, at time T17 the first and second terminal devices have drawing images 12 and 22, respectively.
In the drawing images 12 and 22 of the first and second terminal devices, although the difference in the region b is removed by redrawing, another difference has occurred in the part of the line segment d included in the region e. If the next operation event, for instance, a signal indicating completion of the erasure-type drawing mode (pen off) is thereafter sent from the first terminal device, redrawing is effected in the second terminal device, to remove the difference in the region e. Thus, the display contents of the first and second terminal devices become identical.
However, in the conventional control method described above, as shown in FIG. 1, although accumulated differences are removed by redrawing, there is a possibility that a difference may still exist somewhere as long as there is a terminal device operating in an erasure-type drawing mode.
Further, when the above control method is applied to a drawing communication apparatus dealing with color information, the following problem will occur. For example, as shown in FIG. 2, when a blue horizontal line (light portion in FIG. 2) that is drawn on a drawing screen 10 of a first terminal device and a red vertical line (dark portion in FIG. 2) that is drawn on a drawing screen 20 of a second terminal device intersect each other, a difference that is generated by writing-type drawing cannot be removed.